The present invention relates to an optical modulating device for modulating light incident thereto from the outside.
A gigabit-band ultra-high-speed optical transmission is now being expected to open up great possibilities for one of optical transmission systems which meet with the demand for large-capacity communications such as high-speed data communications and visual communications. In a 1-gigabit or more high-speed optical transmission in a 1.55 .mu.m band in which the transmission loss by optical fiber is minimum, the wavelength dispersion characteristic as well as the loss characteristic of the optical fiber constitute forceful contributing factors to limiting the distance of transmission. On this account, much study has been given so far a dynamic single-wavelength laser which provides a small spectral width during modulation so as to arrest the damaging effect of wavelength dispersion on the distance of transmission.
In the case of direct modulation even by the dynamic single-wavelength laser such as a distributed feedback (DFB) laser, however, a chirping of several angstroms usually occurs, presenting the problem that the received pulse spreads owing to the wavelength dispersion by optical fiber and the chirping. As a solution to this problem there has recently been studied a system in which the output of a semiconductor laser is fixed and held in a state of limited spectral width and is subjected to high-speed modulation by an external optical modulating device.
Optical modulating devices proposed so far are those employing ferrodielectric materials such as LiNbO.sub.3 and semiconductor optical modulating devices of the GaAs series and the InP series which can be integrated monolithically with single-wavelength lasers such as the DFB laser. Of these conventional optical modulating devices, an electroabsorption type one which applies an electric field to a semiconductor waveguide and modulates the intensity of light through utilization of the electroabsorption effect is receiving attention as the most promising device.
In the prior art structure, however, a device capacity increases to disturb a high-speed operation.